Breathing is a remarkable behavior in vertebrates that mediates gas exchange to support metabolism. Failure to maintain a normal breathing rhythm in humans suffering from disorders such as sleep apnea, Rett syndrome, and perhaps sudden infant death syndrome, leads to serious adverse health consequences, even death. Various neurodegenerative diseases, such as Parkinson's disease, multiple systems atrophy and amyotrophic lateral sclerosis are associated with sleep disordered breathing that may result from the specific loss of neurons in brain areas controlling respiration. Control of breathing lies in the brain stem, where the preBotzinger Complex is both necessary and sufficient to generate respiratory rhythm. preBotzinger Complex bursting produces inspiration, while the interburst interval determines expiratory duration. Remarkably, changes in excitability can produce a fifty-fold range of frequencies by specifically tuning the length of the interburst interval. To determine the mechanisms underlying the dynamic modulation of interburst interval by excitability in the preBotzinger Complex, I propose 3 SPECIFIC AIMS-exploiting validated in vitro models of breathing-that will advance our understanding of the neural control of expiratory duration in respiratory rhythmogenesis. AIM 1: While neither pacemakers nor inhibition are essential for rhythm generation in the preBotzinger Complex, these elements may1 play a role in the effects of excitability on interburst interval. I will determine whether blockers of pacemaker currents or inhibition change the response curves. AIM 2: To determine whether the recruitment of active neurons defines the interburst interval and accounts for the cascade of excitation postulated by the group-pacemaker hypothesis, I will monitor the spontaneous activity of preBotzinger Complex neurons using optical imaging of calcium sensitive dyes. AIM 3: The preBotzinger Complex may employ different mechanisms for rhythm generation depending on excitability. I will investigate whether different mechanisms control expiratory duration at high and low frequencies by comparing temporally evolving intrinsic conductances at multiple time points during the interburst interval. These experiments will elucidate the organization and behavior of a fundamental rhythmic neural circuit, necessary for life. Public health relevance - In humans, continuous breathing from birth is essential to life and requires that the nervous system generate a reliable and robust rhythm. The proposed studies will significantly advance our understanding of the neural mechanisms underlying generation of respiratory rhythm and shed light on human disorders of breathing: